This invention relates to a magnetic toner for use in a process for developing latent electrostatic images to toner images with a magnetic brush.
Developers comprising a non-magnetic toner of insulating properties and a magnetic carrier admixed therewith have heretofore been in wide use for the above-mentioned developing process. The magnetic carrier functions to triboelectrify the non-magnetic toner by frictional contact therewith, attracts the toner onto the surfaces of carrier particles to carry the toner to the developing station and serves as a developing electrode. However, the carrier, which is not consumed by development, becomes progressively degraded or the ratio of the toner to the carrier varies, with consequent adverse effects on the developing process.
To overcome this problem, it has been proposed in recent years to use a one-component developer containing no carrier and consisting only of a magnetic toner which per se incorporates a magnetic material, or to use a two-component developer comprising a non-magnetic insulating toner and a magnetic toner admixed therewith, serving as a toner and also as a carrier.
The magnetic toner contained in the one-component developer generally must have a volume resistivity of up to 10.sup.8 ohm-cm to meet the developing requirements, with the resulting problem that the toner images developed can not be transferred to copy paper efficiently or free of disturbance of the image. Thus such toner is not usable for plain paper copiers (PPC) which appear to be copiers of the ideal type. On the other hand, the magnetic toner, when having a volume resistivity of above 10.sup.8 ohm-cm, fails to achieve a high developing efficiency although permitting efficient image transfer and is not suitable for developing latent electrostatic images of low potential. It may be attempted to achieve an improved developing efficiency with the use of a magnet of reduced magnetic force for the magnetic brush developing unit, but this gives rise to many adverse effects. In either case, therefore, the toner is still unfit for use in PPC.
Additionally magnetic toners adapted for use in the one- and two-component developers contain a resin which renders the toner less flowable or conveyable than conventional magnetic carriers. An increased amount of magnetic material, if used in the toner to eliminate this drawback, will result in reduced fixing properties, whereas an attempt to afford enhanced fixing properties will lead to lower flowability or conveyability. The use of the toner thus inevitably involves the sacrifice of either the fixing properties or the flowability (conveyability) of the developer.
It has therefore been desired to eliminate the difficulties which still remain to be overcome despite the attempts heretofore made to solve the foregoing problems.